LTE (Long Term Evolution) is the evolution of 3G (3rd Generation), LTE ameliorating and enhancing the air access technology of 3G, using OFDM (Orthogonal Frequency Division Multiplexing) and MIMO (Multiple Input Multiple Output) as the only standard of the evolution of wireless network. In 20 MHz spectrum bandwidth, LTE can provide the peak rate of downlink is 100 Mbit/s and the peak rate of uplink is 50 Mbit/s, ameliorating the performance of cell edge users, improving cell capability and reducing system delay. The characteristics of LTE include high data rate, packet transmission, low delay, extensive field coverage and downward compatibility. In LTE system, UE (User Equipment) working in one carrier at the same time, while one LTE cell having only one carrier, each LTE cell identified by only one number in network.
In addition, with the rapid growth of the number of mobile user, traffic of mobile user increases exponentially. To satisfy the continuously increased traffic requirements of mobile user, it is need to provide more larger bandwidth for traffic transmission of mobile user and higher peak rate which applications requires. LTE-A (LTE-Advanced) system provides large bandwidth for mobile user, by introducing carrier aggregation technology to improve peak rate for mobile user.
The security of AS (Access Stratum) for LTE system comprise RRC(Radio Resource Control) signalling integrality protection, RRC signalling encryption protection and user data encryption; the RRC signalling is SRB (Signalling Radio Bearer), comprising SRB0, SRB1 and SRB2; the user data is DRB (Data Radio Bearer). SRB1 and SRB2 have the same integrality protection algorithm for RRC signalling integrality protection; encryption algorithm is all the same for all bearers (i.e. SRB1, SRB2 and DRB); integrality protection and encryption operation are not needed for SRB0. Furthermore, the parameters of encryption operation include encryption algorithm and encryption key; the parameters of integrality protection include integrality protection algorithm and integrality protection key.
A network architecture of LTE system as shown in FIG. 1, the network side entity of the LTE system comprise MME (Mobile Management Entity)/S-GW (Serving Gateway), eNB (enhanced Node B) and so on. The interface between eNB and eNB is interface X2. The interface between MME/S-GW and eNB is interface S1. Each eNB includes multiple cells, and each cell can be determined by PCI (Physical Layer ID) of cell and corresponding downlink carrier frequency. For describing conveniently, different cells are determined by {PCI, downlink carrier frequency} in following content. The {PCI, downlink carrier frequency} is the unique identifier in certain field.
Further, because LTE system is covered by cells with single frequency layer, when UE detected a cell corresponding to another {PCI, downlink carrier frequency}, and the signal of this cell is better than that of the serving cell of UE, then it is considered that the UE has moved to the edge of serving cell (cell with better signal). At the moment, it is necessary to perform handover so that the ongoing service won't be interrupted, that is, UE need to move from one cell to another cell.
In LTE system, the above encryption algorithm and integrality algorithm will be changed when UE is switched to another cell from one cell, that is, it is necessary to change the encryption key of RRC signalling, encryption key of user data and integrality protection key of RRC signalling for each handover.
But in present technology, because of the change of AS key (the above encryption key of RRC signalling, encryption key of user data and integrality protection key of RRC signalling) when UE is switched from one cell to another cell, it is necessary to perform re-establishment for PDCP (Packet Data Convergence Protocol) entity, RLC (Radio Link Control) entity, MAC (Media Access Control) entity etc. In the procedure of re-establishing the above entities, it is unable to send or receive data between the network side and UE, thus the procedure of transmitting data may be interrupted and the data may be missing.
In addition, in consideration that each cell only has one (or a pair) carrier in LTE system, and UE only can perform the procedure of data receiving and transmitting in only one cell at the same time, the carrier aggregation technology is adopt to the LTE-A system for supporting wider transmission bandwidth to provide higher transmission rate. There are multiple cells in which UE receives and sends data through carrier aggregation technology at the same time in the same geographic location, and each cell at least include one (or a pair) carrier(s) which can be used to transmit data independently. The multiple aggregated cells can use the same PCI, while the carrier frequency of each cell is different. UE can receive and transmit data in multiple (or multiple pairs) carrier of multiple cell sets in which the carriers can be aggregated at the same time, so as to improve data transmission rate.
As can be seen, the change of the corresponding aggregated cell of UE in the carrier aggregation technology of LTE-A system is not taken into account in present technology, that is, there is not corresponding processing mechanism when the corresponding aggregated cell of UE changes.
To sum up we can see, there are at least the disadvantages below in present technology:
When UE detected the cell with different PCI or frequency, UE considers that the cell with different geography location appears. When the signal of serving cell and other cells with different {PCI, downlink carrier frequency} satisfies handover condition(s), it will perform handover procedure and recalculate AS key and re-establish PDCP, RLC, MAC entities, with the result that too many problems of data transmission interrupt and data loss caused by handover. In addition, there is no mechanism for the change of aggregated cell in present technology.